1. Field of the Invention
The present invention relates to a method for bonding a wire and a capillary tip, and, more particularly, to a method for bonding a metal ball formed on the end of a wire to a material such as an aluminum electrode of a semiconductor chip, and to an apparatus for practicing that method.
2. Description of the Related Art
The method for bonding a wire described above is generally performed by the processes shown in FIGS. 8A to 8E.
After a copper ball 4 is formed by a discharge from a torch electrode 3 on the end of a copper wire 2 extending through a capillary tip 1 (FIG. 8A), the capillary tip 1 is lowered to press the copper ball 4 against an aluminum electrode 8 of a semiconductor chip 7. Chip 7 is attached to a die pad 6 of a copper lead frame 5, and is deformed plastically (FIG. 8B). During this time, the semiconductor chip 7 is heated to a temperature of 300.degree. C. to 400.degree. C. by a heat block 9 on which the die pad 6 is placed Ultrasonic vibrations are applied to the capillary tip 1 by a vibrating device (not shown) to interdiffuse the metallic elements of the copper ball 4 and the aluminum electrode 8, and thereby to bond the copper ball 4 on the copper wire 2 to the aluminum electrode 8.
Next, the capillary tip 1 is raised, which causes the copper wire 2 to feed through the capillary tip 1 (FIG. 8C), and the tip is then moved over and down to the wire connecting surface 11 of the inner lead 110. Wire 2 is pressed against and bonded to the inner lead 10 (FIG. 8D). This is called the stitch bond and looping method. During this time, the inner lead 10 is heated to a temperature of 300.degree. C. to 400.degree. C. by the heat block 9 supporting the inner lead 10. Ultrasonic vibrations are applied to the capillary tip 1 by the vibrating device (not shown) to mutually diffuse metallic elements of the copper wire 2 and the wire connecting surface 11 of the inner lead 10 to produce a bond between the copper wire 2 and the wire connecting surface 11.
Thereafter, the clamper 12 is closed to clamp the wire 2. Next, the capillary tip 1 and clamper 12 are raised, whereby the copper wire 2 is pulled by the clamper 12 to sever the copper wire 2 (FIG. 8E).
The capillary tip 1 employed in the known method of wire bonding performed in the above-described manner has a conical inside chamber 13 which is formed at an angle of 45.degree., as shown in FIGS. 9A and 9B.
FIG. 10 shows the bonding area structure of the copper wire 2 and the aluminum electrode 8 when the copper ball 4 is plastically deformed by the known capillary tip 1. More specifically, the copper ball 4 is plastically deformed due to the loads applied by the conical inside chamfer 13 and a flat loading surface 14 of the capillary tip 1 (FIGS. 9A and 9B). Accordingly, the deformed copper ball 4 has a conical loaded surface 15 and a flat loaded surface 16 (FIG. 10). The lower portion of the copper ball 4 forms a copper and aluminum (Cu-Al) alloy layer 17. An electrode substrate 18 composed of Si or an insulating film is disposed under the aluminum electrode 8.
In FIG. 9A, vectors 19 represent the loads to which the copper ball 4 is subjected as it is pressed. These loads are applied by the inside chamfer 13 and the loading surface 14 of the capillary tip 1. That is, vectors representing the loads applied from the inside chamfer 13 are oriented toward the center of the bonding area, and those representing the loads applied from the loading surface 14 are perpendicular to the aluminum electrode 8. Accordingly, the slip orientation of the copper forming the copper ball 4 is fixed. As shown in FIG. 11, a nucleus 20 having a reduced number of slip-lines is generated in the center of the bonding area on the under surface of the copper ball 4 where the intermetallic alloy of the copper ball 4 and the aluminum electrode 8 is not produced.
The alloy layer 17 cannot be uniformly formed by means of the heat energy applied to the aluminum electrode 8 and the ultrasonic vibration energy applied to the copper ball 4, which reduces the reliability of bonding.
Moreover, recently the circuits of semiconductor chips have become more dense, the electrode size for bonding has shrunk, and smaller metal balls have been required. However, since the orientation of the vectors 19, representing the loads, changes suddenly at a boundary portion 21 between the inside chamfer 13 and the loading surface 14 in the known capillary tip 1 (FIG. 9A), if the size of the copper ball 4 is made smaller, the boundary portion 21 may break through the copper ball 4 and strike the aluminum electrode 8.